1. Field of the Invention
This invention relates to the recovery of sulfur dioxide and more particularly to a new and improved process for recovering sulfur dioxide from gas mixtures containing sulfur dioxide.
2. Description of the Prior Art
Sulfur dioxide has been recovered from gas mixtures heretofore by intimately contacting the gas mixture in an absorber with an organic base, for example an aromatic amine, e.g. dimethyl aniline, as absorbent. The effluent gas containing gaseous aromatic amine from the absorbing step is then scrubbed with a soda solution in a soda scrubber to remove residual sulfur dioxide from the gas as sodium sulfite followed by scrubbing the gas with dilute sulfuric acid solution in a sulfuric acid scrubber to remove the aromatic amine from the gas mixture as the aromatic amine sulfate, e.g. dimethylaniline sulfate. The sulfur dioxide is stripped from the absorbent, and the expelled sulfur dioxide is subsequently recovered in purified, anhydrous, liquefied state. The aqueous effluent solution from the soda scrubber containing sodium sulfite and sodium acid sulfite or sodium and sodium carbonate and the aqueous effluent solution from the sulfuric acid scrubber containing aromatic amine sulfate are combined in a collecting tank, wherein the aromatic amine per se is released from the aromatic amine sulfate and sodium sulfate is formed by reaction of the aromatic amine sulfate separately with the sodium sulfite or sodium sulfite and sodium carbonate present in the combined solutions. The combined liquid in the collectng tank is then passed to a separating tank wherein the released aromatic amine and an aqueous solution layer separate by gravity separation as separate liquid layers. The liquid aromatic amine is withdrawn from its separate layer in the separating tank and passed to the absorbent supply tank for re-use in the absorber for absorbing sulfur dioxide from the gas mixture. Aqueous solution is withdrawn from the aqueous solution layer in the separating tank and passed to the regenerator wherein aromatic amine is released from aromatic amine sulfite contained in the aqueous solution by heating, and steam is generated which is utilized for stripping the sulfur dioxide from the pregnant aromatic amine. Waste aqueous solution containing sodium sulfate is withdrawn from the bottom of the regenerator and consequently considerable quantities of this potentially polluting sodium sulfate-containing waste solution is required to be disposed of without causing undue pollution. As a modification of the aforementioned prior art process, the soda scrubber may be omitted and sodium sulfite or another compound, e.g. sodium carbonate, which will react with the aromatic amine sulfate to result in the release of the aromatic amine and the formation of sodium sulfate may be mixed with the aqueous solution from the sulfuric acid scrubber and a part or all of a water phase separated from the stripped aromatic amine absorbent. The released aromatic amine separates as a separate layer from an aqueous solution layer containing the sodium sulfate and aromatic amine sulfite as aforementioned, and the separated aqueous solution is passed to the regenerator as is hereinbefore described. Considerable quantities of waste aqueous solution containing sodium sulfate are also withdrawn from the bottom portion of the regenerator in this modification, and this considerable sodium sulfate-containing waste solution is also required to be disposed of without causing undue pollution. The aforementioned process and modification thereof are disclosed in U.S. Pat. No. 2,399,013, and although these processes give good results in recovering sulfur dioxide from gas mixtures, the processes leave room for improvement. In the first place, the considerable quantities of waste aqueous solution containing sodium sulfate that are produced are a nuisance and require a considerable monetary expenditure to dispose of such waste solution without undue pollution. Furthermore, considerable amounts of sulfuric acid solution are required to be purchased at considerable expense for scrubbing the effluent gas to remove gaseous aromatic amine.
Other prior art processes for recovery of sulfur dioxide from gas mixtures and which result in the disadvantageous formation of considerable quantities of sodium sulfate-containing waste aqueous solution required to be disposed of, as well as employing sulfuric acid solution as sole scrubbing liquid for removing aromatic amine absorbent from the effluent gas from the SO.sub.2 absorbing step are disclosed in U.S. Pat. Nos. 2,186,453 and 2,295,587.
It is also known in the prior art to use lime for treating the aqueous scrubbing solution from the sulfuric acid scrubber to release the organic base absorbent and to form calcium sulfate. The main problem with the lime treatment is that serious problems are encountered in handling the waste liquid containing the formed calcium sulfate, in that serious scale build up occurs in the process equipment, especially in the heat exchangers, due to the calcium sulfate. Because of this the use of lime for treating the aqueous acid scrubbing solution in the process is undesirable and disadvantageous. U.S. Pat. No. 2,128,027 discloses the use of lime or another calcium compound in the presence of SO.sub.2 and a neutral sulfate, e.g. sodium sulfate, potassium sulfate or ammonium sulfate, for treating the aqueous solution, after separation from the organic base, to precipitate sulfate ions as calcium sulfate, and liberate organic base which immediately forms a soluble sulfite.
U.S. Pat. Nos. 1,972,074 and 2,047,819 disclose washing effluent gas from the SO.sub.2 -absorber tower with sulfuric acid, hydrochloric acid or sulfurous acid to remove organic base absorbent from the gas.